This invention pertains to memories and, more particularly, to a non-volatile memory that utilizes thin film, amorphous transistors, including a floating gate transistor.
Computers currently use two primary types of read/write memory. Specifically, read/write semiconductor memory (commonly called RAM or random access memory) and magnetic disk memory (for example, "hard" or "floppy" disk drives). Read/write semiconductor memory or RAM is characterized by fast access times, high storage density (bits/cm.sup.3), relative high cost per bit of data storage, volatility (volatility is the characteristic whereby data stored in memory is lost as soon as electrical power is removed from the memory) and a lack of moving parts. Magnetic disk memory is characterized by slow access times, low storage density, low cost per bit, non-volatility and moving parts.
Accordingly, the memory described below provides many of the advantages of read/write semiconductor memory, such as fast access time, high storage density and lack of moving parts, with the primary advantage of magnetic disk memory; specifically, non-volatility. In addition, because of the ability to produce this memory on large, single amorphous substrates (rather than the relatively small crystalline substrates of conventional semiconductor devices), this new memory design has the potential to offer a very low cost per bit of data storage.
It is recognized that floating gate transistors have been implemented using crystalline semiconductor material. Such transistors are operated by injecting a charge onto the floating gate by means of an avalanche breakdown of the insulator surrounding the floating gate. Such transistors find use in erasable programmable read only memories (EPROM) where the injected charge can be non-volatilely stored for long periods of time when power is off. However, crystalline floating gate transistor devices have limited use otherwise since it requires a relatively long period of time to erase the charge and reprogram the device, and the insulator has a limited life due to the deleterious effect of the avalanche breakdown.